How do you transfer weight paint in blender 2.8
As of the stable release of 2.80, Transfer Weight option is consolidated into Transfer Mesh Data, and now located under Object > Relations > Transfer Mesh Data. This option is already available in Object Mode, no need to enter Weight Mode and accidentally paint a weight data in the process.

However, the selection order for the Transfer Weight is somehow reversed. So, instead of transferring weight from the first selection to second selection, it's actually vice versa in 2.80 - the tool transfers weight from the second selection to the first selection.